What if this 0-0 boxer defeats the 49-0 boxer who audaciously refers to himself as TBE, an acronym for “The Best Ever,” and who has beaten two men already enshrined in the International Boxing Hall of Fame – and many others sure to be inducted when they’re eligible?

What it would mean for McGregor, the UFC lightweight champion and boxing upstart, is millions upon millions more dollars: Not only for the inevitable rematch, but via endorsements, sponsorships and other fight purses.

Conor McGregor reacts during a media workout in advance of his fight against Floyd Mayweather Jr. (AP)

McGregor won’t have to pick up another bar tab in his life and he’ll go down in history as owner of the biggest sports upset ever.

So long of a shot is he that Leonard Ellerbe, the CEO of Mayweather Promotions and the staunchest Mayweather supporter outside the immediate family, is talking up McGregor’s chances.

“Believe me,” Ellerbe said emphatically to Yahoo Sports, “ain’t no way Floyd is underestimating this guy. All [McGregor’s] losses were on the floor. He’s unbeaten standing up, using his hands. Floyd is taking him very seriously.”

Neither of those matches, though, garnered an extraordinary amount of interest. Few people knew what they were watching when UFC 1 was held in Denver on Nov. 12, 1993. Gracie was unknown to American sports fans and Jimmerson was just a journeyman boxer.

Toney, by contrast, was an ex-world champion and a future Hall of Famer, but he was shop-worn and well past his prime when he met Couture, an ex-UFC light heavyweight and heavyweight champion.

Some may point to the Nov. 14, 2015, UFC championship match between then-women’s bantamweight champion Ronda Rousey and ex-boxing champion Holly Holm as another example. Holm scored a stunning second-round knockout, and many in boxing hailed it as an example of boxing’s superiority.

That view, though, doesn’t quite jibe with the facts. By the time she’d met Rousey, Holm had been training MMA exclusively for more than five years and was a full-fledged mixed martial artist.

For years, a large swath of MMA fans have sought acceptance for their sport, as if they needed approval from sports fans writ large. Much of the disdain toward MMA from boxing fans was ignorance of the sport.

MMA fighters take different stances and throw more looping punches, but it’s because of all they have to defend against. Boxers and boxing fans see stand-up in MMA as simply as bad boxing.

A McGregor win would resoundingly provide that acceptance which many MMA fans have desired.

“Oh my God, it would blow up more so than it is now,” said Javier Mendez, a leading trainer at the American Kickboxing Academy in San Jose, California, a top MMA gym. “It’s almost incalculable how much it would mean. If Conor does this, well, let’s be honest. Realistically, it shouldn’t even go two minutes. He shouldn’t even go two minutes with a great one like Mayweather. He really shouldn’t. In my mind, if I’m the great one, I’m going to say, ‘This guy is an amateur, and I’m going to take him out right away.’ I don’t see how it benefits Mayweather to go 12 rounds with McGregor. Seven rounds would be a loss. Six rounds would be a loss. Anything less than an annihilation would be a loss for Mayweather.

“But if Conor does it, oh my God, think about it. It will boost the money in the sport. Anyone who fights Conor would make so much more. It would lift the money all around, but especially for guys like Jon Jones and Daniel Cormier who already have a following. It would bring a lot more eyeballs to MMA because the boxing people would see it and say, ‘Hey, these guys can fight, too.’ It would be huge for MMA if he did it.”

It would be a final validation, of sorts, as a clear McGregor win would provide conclusive proof that MMA fighters are quality athletes in their own right. That should be plainly obvious by now, but there remain a sizeable percentage of fans who still haven’t followed MMA closely – or at all. The sheer number of fans who still haven’t learned about MMA is a huge growth opportunity for the sport’s top promoters.

The perception has long been that MMA fighters are generalists, good at many disciplines but frequently not great at any one of them. UFC Hall of Famer Urijah Faber said a victory by McGregor over Mayweather would be evidence that theory is not exactly right.

The top fighters, he said, are elite in all aspects of the game.

“If he wins, it would be massive for us in so many ways, but for the fighters, it would slam the door on a lot of arguments,” Faber said. “It would show that we as MMA fighters have mastered all the disciplines. … I could compete in jiu-jitsu with world-class jiu-jitsu players. I could wrestle with wrestlers, box with the boxers.

“That would be big for the fighters, but it would be significant on a lot of levels business-wise, too. MMA has been on the rise and is going to continue to grow, but if Conor wins, I think you’d see it would accelerate that growth even more quickly.”

McGregor scoffs at talk that he is fighting an uphill battle against Mayweather, and insists that most of the advantages in the fight belong to him. Even when it comes to boxing footwork, at which Mayweather is an acknowledged master, McGregor has a ready response.

“I’ll go out and I’ll paint many different pictures,” McGregor said. “I can fight in many stances. I can fight in many styles. I can take a capoeira stance. I can take a taekwondo stance. I can take a karate stance. I can take a boxing stance. I can take a Greco Roman freestyle-wrestling stance. I can stand and fight in many, many ways, and I will fight in all of them.

“I will paint many, many different pictures in this fight. I’ll dazzle him, and then the shots will find their home.”

The television audience for the Mayweather-McGregor fight is not going to be made up solely of fight fans, of either boxing or MMA. There will be many people curious who haven’t been devoted to the fight game who will tune in because of the extraordinary attention the match is receiving.

If McGregor wins, and puts on an entertaining show as he almost always does before, during and after the fight, he’ll undoubtedly convert a solid percentage of those into MMA fans.

It would lead to more regular pay-per-view sales for the UFC, as well as more and higher-paying sponsors. Some sponsors who refuse to advertise during UFC broadcasts would probably be compelled to re-think that argument.

A McGregor win over a man considered one of the greatest boxers of all-time would do nothing but improve the stature of the UFC. It would almost be a license for it to print more money.

But Dominick Cruz, a former UFC bantamweight champion who will work the fight as an analyst for Fox and Fox Sports 1, believes there will be more super fights between boxers and MMA fighters in the near future because of what McGregor has done.

He doesn’t even have to win – or look good in losing – to do it, either, Cruz said. McGregor has brought MMA to parity, and the big fights between athletes in the two sports are inevitable.

“I think we’re there now,” Cruz said. “We’re seeing the level of MMA raise, as well. That’s the other issues boxers don’t get. There is no place to put a wall or a level in terms of how good MMA fighters can get, because we’re still trying to grow the sport so much. We’re still trying to bring the money up and not just that, but bring everything up that is involved with mixed martial arts. We’re just going to keep improving, improving, improving, improving.

“When you look across the board at MMA, it comes down to us athletes trying to be perfect in each martial art. That takes time, effort, good coaching and the sport evolving so we have teachers to train these guys to be that good at boxing, to be that good at jiu-jitsu, to be that good at wrestling and be that good at mixing it all together.”

It’s a no-lose situation for not only McGregor, but also UFC honchos Ari Emanuel, Patrick Whitesell and Dana White. If McGregor loses, he was expected to, and it should have no discernible impact upon their business.

If he wins, though, it would unlock the keys to the vault.

White is a gambling man, but this match is the safest gamble he and his partners could have made. There is little downside and the upside is enormous. Those are the bets White wishes he could get at the many local casinos he frequents.